WASHINGTON — Ask officials at the White House to pinpoint the debt ceiling deadline and they will give you a fairly precise answer: Thursday.

But ask them when a failure to increase the debt ceiling will start to hurt — when the bills will start going unpaid and America’s creditors will start calling in their debts — and just about everyone in President Obama’s administration will quickly clam up.

“For details on what would happen if we were to cross that line, I would refer you to the Treasury Department,” Jay Carney, the White House press secretary, told reporters on Tuesday. But Treasury Department officials declined to provide any information beyond what Secretary Jacob J. Lew told Congress at a hearing last week — which was not much.

The reluctance to provide details is simple: no one knows what will happen on Friday or Saturday or Sunday. In modern times, the United States government has never defaulted on its debt or failed to raise the debt ceiling.

The federal government ran out of its authority to borrow money months ago. Since then, the Treasury Department has been juggling funds with what officials call “extraordinary measures” designed to pay the bills. Mr. Lew says he will no longer be able to do that after Oct. 17.

The stock market, which has remained relatively stable so far, could plummet. The buyers of Treasury bonds could demand much higher prices, raising the amount of cash that the government would need to pay out. And the United States could face unexpected expenses — the way many families do — that will put pressure on the Treasury.

But there are also political reasons why the White House might not want to be too forthcoming about the due dates for government bills. Any suggestion that major debts will not need to be paid for days or weeks could relieve the pressure on Republicans to agree to raise the debt ceiling without conditions.

Many of the Democrats’ warnings have been built around the idea that missing the deadline on Thursday will spell doom for the United States and the global economy. After meeting with Mr. Obama on Tuesday afternoon, Representative Nancy Pelosi of California, the Democratic leader, was asked by a reporter whether Americans would even notice any immediate changes.

“Oh, please. I mean, we’re talking about something so catastrophic,” Ms. Pelosi said. She called the idea of barreling past the deadline “insanity.”

Photo

Jay Carney, the White House secretary spoke about the shutdown agreement during a White House briefing on Wednesday.Credit
Stephen Crowley/The New York Times

Some market observers have suggested that the government might have enough money to pay its bills for several days, noting that it must make a $12 billion Social Security payment on Oct. 23 and a $6 billion interest payment on the public debt on Oct. 31. On Nov. 1, the government must spend $58 billion for Medicare and Social Security payments, military salaries and other expenses, according to the Bipartisan Policy Center, a research group based in Washington.

But White House officials declined to confirm those dates or to offer any information about what they expected to happen if lawmakers did not reach a deal by Thursday. Asked when it would be “too late” for Congress to act, Mr. Carney, the White House press secretary, demurred.

“I’m not sure what that means,” he said. “They need to act as soon as possible because what is absolutely true is that every day we’re in shutdown there is harm done to hundreds of thousands of Americans and indirectly to many, many more. And there’s direct harm done to our economy. And every day we get closer to the point beyond which we have never been.”

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Administration officials often point out that the damage from the high-stakes standoff has already begun. Short-term Treasury bills have become more expensive for the government to issue, as bond owners demand higher interest rates in light of what they see as a riskier investment proposition.

And on Tuesday, the Fitch credit rating agency put the country on a watch, essentially threatening to downgrade America’s creditworthiness based on a belief that the political system has ground to a halt. That could mean a downgrade is coming even if lawmakers reach a last-minute deal.

Some Republican lawmakers and pundits have said the government can avoid serious economic damage by deciding to use its cash on hand to make the interest payments on its debt, even if that means choosing not to make automatic payments to Social Security and Medicaid recipients or to satisfy other nondebt obligations.

White House officials scoff at such ideas, in part because of the political nightmare that such moves would create. But they also note that global investors who are considering whether to buy American bonds might get spooked by the thought of investing in a country that is willing to renege on such big promises to its elderly and poor.

In the end, though, the honest answer may be that they just do not know. “The very uncertainty that has been created by this manufactured crisis is what we need to avoid as an economy going forward,” Mr. Carney said Tuesday. “It is already causing uncertainty among Americans, which in turn has them making decisions about how they spend their money, which has a negative impact on — potentially on the economy. And that creates a cascading effect that can only be bad.”

Asked about cable news programs that are running countdown clocks ticking down to the deadline, Mr. Carney shrugged.

“As much as I’d like to improve the quality of the countdown clocks,” he said, “I would have to refer you to Treasury on the minute and the hour.”

Correction: October 16, 2013

An earlier version of the picture caption with this article misstated the date the photo was taken. The photo of is Jay Carney’s White House press briefing on Wednesday, not Tuesday.